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 Post subject: Proxies and database hit
PostPosted: Sun Nov 02, 2008 1:13 am 
Newbie

Joined: Tue May 27, 2008 7:11 am
Posts: 5
Hello,

I have a general understanding of Hibernate but I'm more interested in cashing capabilities, so I've started from Ch.13. But the following example confused me a little bit:

ch13, p565:
Quote:
A proxy is useful if you need the Item only to create a reference, for example:
Code:
   Item item = (Item) session.load(Item.class, new Long(123));
   User user = (User) session.load(User.class, new Long(1234));
   Bid newBid = new Bid("99.99");
   newBid.setItem(item);
   newBid.setBidder(user);
   session.save(newBid);

You first load two objects, an Item and a User. Hibernate doesn’t hit the database
to do this: It returns two proxies. This is all you need, because you only require
the Item and User to create a new Bid. The save(newBid) call executes an INSERT
statement to save the row in the BID table with the foreign key value of an Item
and a User—this is all the proxies can and have to provide. The previous code
snippet doesn’t execute any SELECT!


As far as I understand, session.load is the Hibernate way to retrieve by primary key. If the previous snippet doesn't execute any SELECT, how Hibernate knows that an Item entity with id 123 exists?


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 Post subject:
PostPosted: Sun Nov 02, 2008 2:45 am 
Hibernate Team
Hibernate Team

Joined: Mon Aug 25, 2003 9:11 pm
Posts: 4592
Location: Switzerland
It doesn't. It just generates an instance of Item with that identifier value. We all hope that it really exists in the database as a row. If it doesn't, you will get an exception at some point, in the code example as soon as you try to save the Bid instance. The foreign key reference to the item identifier does then not resolve.

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 Post subject:
PostPosted: Sun Nov 02, 2008 2:57 am 
Newbie

Joined: Tue May 27, 2008 7:11 am
Posts: 5
Thanks.


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